Polypropylene homopolymer has many useful applications. However, polypropylene homopolymer alone is often unsuitable for applications which require low melting point and more flexibility as well as enhanced clarity. Polypropylene random copolymers (RCP) are specially suited for such applications. Conventional RCPs are typically made through random incorporation of ethylene or other comonomer into polypropylene. The presence of comonomer disrupts polymer stereoregularity and lowers its crystallinity, resulting in lower melting point, lower modulus and higher clarity.
A large number of processes for preparing propylene homo- and copolymers are known in the art. Many different kinds of slurry and gas phase processes can be employed when a supported catalyst is used for polymerization. The bulk process is a slurry process, wherein the reaction takes place in pure monomer or in a reaction medium containing more than 60 weight % of the monomer. The bulk process is carried out in either continuously stirred tank reactor (CSTR) or loop reactors. In a loop reactor, the first reaction stage consists of one or two tubular loop reactors where bulk polymerization of homopolymers is carried out in liquid propylene. The prepolymerized catalyst, liquid propylene, hydrogen for controlling molecular weight are continuously fed into the reactor in which polymerization takes place at temperatures of 60-80° C. and pressures of 35-40 bar. The polymer in liquid propylene inside the loops is continuously discharged to a separation unit. Unreacted propylene is recycled to the reaction medium. The granules are discharged to a flashing unit for product/monomer separation. One difficulty associated with slurry processes is fine particle generation. This is especially true for the production of high melt flow rate (MFR) polypropylene.
Random copolymers produced during bulk/slurry polymerizations using hydrocarbon solvents, in particular polymers of high ethylene content and/or low molecular weight, are sticky in the reaction medium. This can cause considerable problems in such bulk/slurry polymerization applications. This problem can be mitigated by operating the polymerization reactor under super critical conditions as disclosed in WO 92/12182. By nature the super critical fluid has lower solvency to polymer, and nearly unlimited solubility of gaseous components. Simultaneously, the separation of the recycled reaction medium and recovered polymer is simplified, because of the energy available in the polymerization product. However, supercritical operation requires handling of high-pressure equipment and is energy intensive and expensive.
Production of high ethylene content and/or low molecular weight polymers also causes difficulty in the operation of conventional flash systems. Such flash systems are highly sensitive to highly soluble polymer fractions. Any non-evaporated liquid in the separation tank risks blocking the device. This is particularly true for cyclone type of devices operated at high pressures.
The stickiness of polymer can be mitigated through reducing the granule swell and improved particle morphology. An example of a polymerization process that incorporates the use of a nonreactive diluent is shown in U.S. Pat. No. 3,470,143 (Schrage et al.). Specifically, the Schrage patent discloses the use of a fluorinated organic carbon compound as a diluent in polymerizing at least one ethylenically unsaturated hydrocarbon monomer to form an amorphous elastomer. The product can be dried in the form of small particles.
EP 1 323 746 shows loading of biscyclopentadienyl catalyst onto a silica support in perfluorooctane and thereafter the prepolymerization of ethylene at room temperature.
U.S. Pat. No. 3,056,771 discloses polymerization of ethylene using TiCl4/(Et)3Al in a mixture of heptane and perfluoromethylcyclohexane, presumably at room temperature.
U.S. Pat. No. 5,624,878 discloses the polymerization using “constrained geometry metal complexes” of titanium and zirconium.
Adhesion of polymers to reactor walls in slurry polymerization processes is and has been a known problem. Japanese Kokai Patent Application No. SHO 61[1986]-7301 indicates that a prior method of reducing this adhesion or fouling problem was to keep the slurry concentration at a relatively low level. However, such a process would have to be run at a relatively low polymer productivity. A further method of reducing polymer adhesion that is also described in the ′7301 Kokai is to use a certain concentration of fluorocarbons in the polymerization system. The amount of fluorocarbon used in the system is generally limited from 0.01-5weight %. Below this amount, the use of the fluorocarbon is said to be ineffective and above this amount is reported to result in lower polymerization activity.
There remains a need to increase polymer product quality and process efficiency, particularly processes that reduce slurry polymerization fouling without suffering any substantial loss in polymerization activity. It is particularly desirable to find polymerization processes that use propylene as at least one monomer feed component, and to produce a polypropylene type product that can be recovered in particle form. Such a process would also be desirable in the production of polypropylene type polymers having low crystallinity. Processes that provide for higher flexibility in types of catalyst that can be used, as well as the flexibility to use lower quality propylene feeds, are especially preferred.